Corrosion: Causes, Effects, and Types

Corrosion

• Corrosion happens when metals deteriorate due to chemical or electrochemical reactions with the environment.
• Examples include rusting of iron and the formation of green film on copper.

Causes of Corrosion

• Metals in their pure state are thermodynamically unstable and have a natural tendency to return to their more stable state.
• This happens through reactions with the environment which form surface compounds.

Effects of Corrosion

• Corrosion degrades the physical properties of metals like malleability, ductility, and electrical conductivity.
• Reduces the efficiency of metallic structures.
• Contaminates products if corroded equipment is used.
• Requires time-consuming and costly repairs and replacements.

Types of Corrosion

• Dry or Chemical Corrosion: occurs under dry conditions due to the direct reaction of gases like oxygen, chlorine, carbon dioxide, sulfur dioxide, nitrogen oxides, and anhydrous liquids with the metal.

  • Oxidation Corrosion: Oxygen reacts with metals at ambient temperature in the absence of moisture.
    • Alkali and alkaline earth metals corrode at room temperature while high temperatures cause oxidation in most metals except for silver, gold, and platinum.
    • The nature of the oxide layer formed influences further corrosion:
      • Stable oxide layers protect the metal from further attack (e.g., oxides of lead, aluminum, and tin).
      • Unstable oxide layers decompose back into metal and oxygen, preventing oxidation corrosion (e.g., noble metals like gold, silver, and platinum).
      • Volatile oxide layers evaporate as soon as they are formed, exposing the underlying metal for further attack (e.g., molybdenum forms volatile MoO3).
      • Porous oxide layers allow atmospheric oxygen to reach the metal surface through pores, causing continuous corrosion (e.g., alkali and alkaline earth metals).
  • Pilling-Bedworth Rule:
    • If the volume of the metal oxide formed is greater than or equal to the volume of the metal, the oxide layer is protective and non-porous.
    • If the volume of the metal oxide formed is less than the volume of the metal, the oxide layer is non-protective and porous.
  • Corrosion by other gases: Gases like chlorine, sulfur dioxide, hydrogen sulfide, and nitrogen oxides react with metals to form protective or non-protective corrosion products.
    • Chlorine reacts with silver to form protective AgCl, but with tin, it forms non-protective SnCl4.
    • Hydrogen sulfide attacks steel at high temperatures in petroleum industries, forming porous FeS scale.
  • Liquid Metal Corrosion: Occurs when a liquid metal flows over a solid metal at high temperatures (e.g., sodium metal corrodes cadmium rods in nuclear reactors).

• Wet or Electrochemical Corrosion: Happens under wet or moist conditions, especially when a metal is in contact with a conducting liquid, or dissimilar metals are partially submerged in a corrosive environment.

  • Requires the presence of anodic and cathodic areas, a conducting medium (electrolyte), and a potential difference between the anodic and cathodic areas.
  • Factors affecting wet corrosion:
    • Nature of the metal:
      • If the corrosion product is unstable, corrosion does not occur.
      • If the corrosion product is porous or volatile, severe corrosion happens.
      • If the corrosion product is soluble in the medium, corrosion happens at a faster rate.
    • Nature of the environment:
      • Humidity: The higher the humidity, the faster the rate of corrosion.
      • Temperature: Higher temperatures increase the rate of corrosion.
      • pH: Metals corrode faster in acidic pH than alkaline or neutral pH. Amphoteric metals like aluminum and zinc corrode in basic mediums.

Corrosion Control Methods:

• Cathodic Protection: Forces the metal to be protected to behave like a cathode, preventing corrosion.
  • Sacrificial Anodic Protection:
    • Connects the metallic structure to be protected to a more active metal (sacrificial anode).
    • The more active metal corrodes slowly, protecting the structure by acting as an anode.
    • Common sacrificial anodes include magnesium, zinc, aluminum, and their alloys.
    • Used for buried pipelines, underground cables, marine structures, ship hulls, and metallic water tanks.
  • Impressed Current Cathodic Protection: Uses an external power source to create a current that flow from the anode to the cathode (the metallic structure to be protected).
• Protective Coatings: Apply a barrier layer to prevent direct contact between the metal and the environment.
  • Metallic Coatings: Zinc plating (galvanizing), tin plating (tinning), chromium plating, nickel plating, etc.
  • Organic Coatings: Paints, varnishes, lacquers, etc.
• Inhibitors: Chemical substances that slow down the rate of corrosion by interfering with the corrosion process.
  • Anodic Inhibitors: Form a protective film on the metal surface.
  • Cathodic Inhibitors: Reduce the rate of the cathodic reaction.
  • Mixed Inhibitors: Act on both anodic and cathodic reactions.
• Alloying: Combine two or more metals to create an alloy with improved corrosion resistance.
• Design considerations:
  • Avoid sharp corners and crevices.
  • Minimize the use of dissimilar metals.
  • Use corrosion-resistant materials.

Electrodeposition

• Electroplating: Depositing a desired metal on the surface of another metal using an electrolytic cell.
  • Article to be plated is made the cathode, while the anode is either the metal to be deposited or an inert material.
  • Metal ions migrate to the cathode and get deposited.
  • Electroplating enhances corrosion resistance, wear resistance, and imparts decorative finishes.
• Electroless Plating: Depositing a metal from its salt solution on a catalytically active surface using a chemical reducing agent without electricity.
  • The process requires a pre-treatment step to activate the surface of the base metal.
  • The plating bath contains the metal salt, a reducing agent, complexing agents, exaltants, stabilizers, and buffers.
  • Electroless plating is used for various applications, including circuit boards, decorative surfaces, and functional coatings.